The present invention relates to a ball end mill used to machine ball grooves of constant-velocity joints, etc. and particularly to the shape of cutting blades of a ball end mill made of a polycrystalline hard sintered material containing cubic boron nitride.
Materials formed by sintering fine cubic boron nitride powder by use of various kinds of binders exhibit excellent properties for the machining of high-hardness iron-base metals and cast iron. If a cutting tool made of a cubic boron nitride sintered material is used for the machining of a hardened steel material which has an especially high hardness, finished surface roughness and dimensional accuracy equivalent to those by grinding are obtained. Thus, part of the cutting of hardened steel is being changed from cutting using a conventional grinder to cutting using a cutting tool of cubic boron nitride sintered material.
In contrast, the field in which precision cutting is the most advanced is in the cutting of non-ferrous metals using single-crystal diamond. Its major application is machining of aluminum alloys such as memory disk boards and polygonal mirrors. Recently, there is a tendency to replace such single-crystal diamond with a less expensive and easy-to-machine polycrystalline diamond. With polycrystalline diamond tools, if large irregularities are formed at the ridge of a cutting blade due to steps at the crystal interface, they cannot be removed even if the rake face and flank are ground. Thus such irregularities remain on the ground surface and are transferred onto the workpiece. Thus no accurate machining was possible.
With a polycrystalline diamond cutting tool disclosed in Japanese patent publication 6-190610, this problem is solved by chamfering its cutting edge. It discloses forming the flank of the cutting edge of a cutting insert made of polycrystalline diamond into a ground surface having no grinding marks, and providing a small-width chamfer on the ridge of the cutting blade by grinding to smoothen the ridge of the cutting blade. But with a polycrystalline diamond tool, since carbon forming the diamond reacts with steel, it cannot be used for the cutting of hardened steel and cast iron.
The housing and ball grooves of a constant-velocity joint made of carbon steel such as S50C are, after rough machining, subjected to heat treatment to a hardness of HRC 50-60. The ball grooves, which are formed diagonally in the housing, are required to be formed to narrow geometric tolerance and sphericity with high accuracy by finishing. Even if the material and shape of the cutting blades of a ball end mill correspond to a hard, heat-treated workpiece, a side lock type ball end mill is not preferable which is joined to a collet holder as shown in FIG. 5.
FIG. 5A is a sectional view of the joint portion with a collet holder, and FIG. 5B shows a section along Xxe2x80x94X line of FIG. 5A. In FIG. 5A, a ball end mill body 100 has its shank 101 inserted in a clamp hole of a collet holder 102 and fixed by a set bolt 104. But the shank 101 is pushed to one side of the clamp hole 103 by the set bolt 104 as shown in FIG. 5B, so that the axis of the ball end mill body 100 is offset by a distance S. Even if the offset amount S is as small as several micrometers, it is multiplied when measured from the axis of the collet holder 102. Thus, this arrangement is not suitable for high-precision machining for forming ball grooves.
An object of the present invention is to improve the surface roughness Rz of a cut surface to 1.6 xcexcm or less in cutting hardened steel by use of a polycrystalline hard sintered member, i.e. to achieve excellent finished surface roughness and high dimensional accuracy by improving the shape and surface roughness of a cutting edge of a cutting tool made of polycrystalline hard sintered material containing cubic boron nitride. Rz indicates a 10-point average roughness specified in JIS B0601 xe2x80x9cSurface roughnessxe2x80x94definition and indicationxe2x80x9d.
Further, in order to achieve such dimensional accuracy, it is necessary to improve a means for coupling the ball end mill body with a collet holder. If shrink fit joint is used for coupling, a material which is high in rigidity and small in thermal expansion coefficient should be used for the shank portion.
A polycrystalline hard sintered material containing 20-95 vol % of cubic boron nitride is used as the cutting blade of a ball end mill. The cutting edge ridge is subjected to honing to a radius of curvature of 5 to 30 xcexcm. A flank and a rake face or a negative land of the cutting blade smoothly connect with the ridge with the radius of curvature. The ridge of the cutting blade has a surface roughness of 0.1 to 1.0 xcexcm in terms of 10-point average roughness (Rz).
Preferably, the flank and the rake face or the negative land of the cutting blade has a surface roughness of 0.1-0.5 xcexcm in terms of 10-point average roughness (Rz). The wedge angle between the flank and the rake face or between the flank and the negative land is preferably 65xc2x0 to 125xc2x0.
As the polycrystalline hard sintered material, one containing 20 to 95 vol % of cubic boron nitride and having an average particle diameter of 0.01 to 5 xcexcm is preferable. A coating layer comprising at least one of a nitride, carbide, oxide and a solid solution thereof of at least one element selected from the group consisting of elements in the 4a, 5a and 6a groups of the periodic table and elements of Al, Si and B, or at least one of metal selected from the group is formed on the surface of the cutting blade by physical or chemical deposition.
A cemented carbide or a sintered superheavy alloy is used for the ball end mill body. The shank portion has such a shape as to be shrink-fit into a collet holder to minimize runout of the cutting blade and strongly join them together by utilizing the difference in thermal expansion coefficient between the shank portion and the collet holder.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: